Okay, we weren't sure about taxation when we said this but it turns out we were right anyway, at least the most part. I did some research this evening and here's how taxation works:

Taxation research does two things (and only two things): 1) generates wealth intrinsically based on your territory, and 2) increases the value of all rare resources (including fishing).

Wealth generated from territory is based solely on the percentage territory you control, and not on any other factors.
At level 1, you get +0.5 wealth per 1% of territory. (Thus the 50% rate it advertises). British get +1.
At level 2, you get +1 wealth per 1% of territory. British get +2.
At level 3, you get +2 wealth per 1% of territory. British get +4.
At level 4, you get +3 wealth per 1% of territory. British get +6.

For each level of taxation the income from all rare resources increases. However, for fishing, only the non-food portion of the fisherman's income is increased. Also, only the base amount is used for the bonus calculation. (i.e., Nubians do not get extra bonuses from researching taxation.)
At level 1, you get a 20% increase in the rare resource (+10 becomes +12).
At level 2, you get a 50% increase in the rare resource (+12 becomes +15).
At level 3, you get a 100% increase in the rare resource (+15 becomes +20).
At level 4, you get a 200% increase in the rare resource (+20 becomes +30).

While we're on the subject of rare resources, the Porcelain Tower increases the (base) income from rares by 200% (again this is additive, not multiplicative). So that juicy Nubian rare which is netting +35 does not go up to +105, which is what would make sense: 35 + 2*35 = 105. Instead, the base of +10 is what is used, so an additional +20 is added. Capped out, with the tower, for a Nubian, a +10 rare brings in +55. For mere mortals, it would bring in +50.

One correction to the earlier post, Nubians do not receive any kind of bonus from fishing.